Refactor the SplitViewController

Figured out the constructor for the UIViewController. Moved alloc and
initialization functions into the constructor.

Issue #5
Implement a collection of TimeSheets

Implementation is done using Categories. More can be found here:
http://stackoverflow.com/questions/3862619/should-i-subclass-the-nsmutablearray-class

Issue #5
Store TimeSheets in the SplitAppDelegate

Additionally provide the capability to store the active TimeSheet in the
SplitAppDelegate. This allows the application to track which TimeSheet
the user was last viewing.

Issue #5
Dead code removal
Refactor the update update code

Allows 1 method to be called and makes the entire UI reflect the data
from the associated data model.

Issue #5
Squashed bug with TimeSheet#getElapsedTimeInterval

TimeSheet#getElapsedTimeInterval was never referencing the stopTime
parameter. This was causing the display to continue to update if the
SplitViewController was loaded and unloaded.

Issue #5